1. Field of the Invention
The present invention relates to stabilization of inorganic pigments in aqueous media. More specifically the invention relates to stabilization of inorganic pigments with acid functional polyester resins or phosphate esters.
2. Description of the Related Art
Inorganic pigments are included in waterborne coating compositions as the pigment provides desirable color effects. Various problems are encountered in waterborne coatings containing these pigments, when the pigments are utilized without stabilization. These problems include gassing or oxidation of the inorganic pigments when the pigment is exposed to an aqueous environment. The pigment reacts with water to produce hydrogen gas and a metal hydroxide. The reaction is a form of corrosion. The amount of corrosion is measured by the amount of hydrogen gas produced over a given period of time. Corrosion is exacerbated since the reaction results in the continuous formation of H.sup.+ and OH.sup.- ions. The H.sup.+ ions attack and corrode the pigment and the OH.sup.- ions cause the pH of the environment to further increase. In coating compositions, contact of the inorganic pigment with the aqueous environment is continuous over extended periods of time, since coatings containing the pigment are often stored for 6 months or more before use.
Gassing of the inorganic pigment causes a loss of viscosity of the coating composition and consequent loss of metal control in the coating composition. The result is often a cured film exhibiting a gray color, rather than the desired bright metallic appearance.
An object of the present invention is treatment of inorganic pigment to reduce or eliminate pigment gassing, when the pigment is used in an aqueous environment or in a waterborne coating composition.